Tagged-analyte complexes are fragmented in a mass spectrometer to generate a charged signature ion. The signature ion is used to quantify the tagged analyte. Unfortunately, during the fragmentation process, positive charge is predominately transferred to the signature ion and the remaining charge-neutral portion of the molecule containing the analyte remains undetected or under-detected. Subsequent analysis for fragmentation of the charged neutral portion containing the analyte does not lead to unambiguous identification of the analyte. A need exists for a method for the simultaneous quantitation and identification of small molecules, for example, metabolites.